Band pass filter



Dec. 4, .1956 D. D. GRIEG 2,773,244

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BAND PASS FILTER Filed Aug. 2. 1952 3 SheetsSheet 2 Dec. 4, 1956 D. D. GRIEG BAND PAss FILTER 3 Sheets-Sheet 3 Filed Aug. 2, 1952 N= ODD INTEGER INVENTOR DONALD /f BY A TORNE United States Patent() BAND PASS FILTER Donald D. Grieg, North Caldwell, N. J., assiguor to International Telephone and Telegraph Corporation, a corporation of Maryland Application August 2, 1952, Serial No. 302,292

1 Claim. (Cl. S33- 73) This invention relates -to bandpass filters and more particularly to tunable microwave bandpass filter systems for utilization in heterodyne receivers or similar equipment to provide ease of tuning the filter and associated equipment over a wideband of microwave frequencies.

In microwave receivers the need for an etiicient and practical microwave filter has always been considered of paramount importance. The principal purpose of such a filter is to eliminate the possibility of receiver responses to an image signal which could either interfere with the desired signal or completely capture a `receiver utilizing limiter stages as are commonly found in communication receivers. The problem resolves itself into the construction of a microwave circuit which provides the required attenuation at the image frequencies, while yet passing the desired signal spectrum free from attenuation and distortion. Furthermore, since the microwave receivers presently being employed are required in many applications to be operable over a large frequency range it is imperative that the filters employed in conjunction therewith be tunable over the desired frequency band and have characteristics which are uniform over this band. These characteristics include the width of passband, image attenuation, insertion loss, and accuracy of tuning.

The general application of microwave filters has been widely explored by many investigators in thefield with particular regard to filters which operate over a narrow range of central frequencies. A typical filter design is an alignment of tunable cavities in a straight section of waveguide, with a separation of a quarter wavelength between adjacent cavities. In order to tune a filter of this type over a band of frequencies many mutually coupled parameters must be adjusted. There is the resonant frequency tuning adjustment for each cavity, the passband shape adjustments, and often adjustments between adjacent cavities must be provided for. In a typical three cavity coupled filter as many as nine mutually dependent adjustments may be required in order to tune to an adjacent frequency.

A modern microwave receiver consisting of a preselector filter unit for providing the required image rejection and selectivity plus a mixer or converter, a local oscillator, and I. F. amplifier, and the remaining units making up the conventional heterodyne receiver must each be tuned separately to tune in the desired signals since Vit is not practical to gang all these tuning elements with sufficient accuracy. Therefore, it is extremely difiicult when hunting for a signal to insure that all the receiver components or units are appropriately aligned and to maintain the alignment with the result that the tuning and alignment procedure required by such a receiver is reduced to a trial and error method. This often results in an inability to locate a given signal or the expenditure of considerable time and effort to do so. If the filter is reasonably selective as is required for good image rejection, the tuning problem becomes particularly onerous.

l With the difficulty of receiver tuning as observed above and the complexity of the preselector lter tuning known in the prior art, it is an object of this invention to provide for a microwave bandpass filter a means to partially shunt the signal from said filter allowing for simplification of the overall receiver tuning procedure.

Another object of this invention is a retractable probed shunt which when inserted into the signal energy path does not disturb appreciably the signal propagation mode of the filter and permits the signal energy to be partially bypassed around the filter unit to reduce the filter selectivity and thereby provide an opportunity to broadly align and tune the filter unit as well as the remaining units of a conventional microwave receiver of the heterodyne type, and when retracted from the signal energy path the receiver is substantially selectively aligned with little further tuning adjustments necessary.

A feature of this invention is the insertion of a retractable probed coaxial shunt into resonant waveguide filter sections as an integral portion of a tunable waveguide filter for shunting a portion of the signal energy from said tunable filter section.

Another feature of this invention is the insertion of a retractable probe coaxial shunt into waveguide sections adapted to be connected prior to and following the main tunable waveguide filter sections.

Still another feature of this invention is the employment of a coaxiai section prior to and following a coaxial type microwave filter wherein the signal energy is coupled capacitively to and from a transmission line.

A further feature of this invention is the employment of means for controlling remotely the operation of the bypass communication.

Still a further feature of this invention is the employment of a probed transmission line wherein the energy flow through said transmission line is adjusted to control the operation of the bypass communication.

The above mentioned and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawings, in which:

Fig. l is a block diagram of a heterodyne receiver employing an embodiment of the bandpass filter following the principles of this invention;

Fig. 2 is a plan view, partially in section, of the preselector filter of Fig. l incorporating an embodiment of the bypass means of this invention;

Fig. 2a is a graphical representation of the method for tuning a microwave receiver when employing an embodiment of this invention;

Fig. 3 is a sectional view of another embodiment of the bypass means arranged for coupling with a given preselector filter;

Fig. 4 is a sectional view of still another embodiment of the bypass means in accordance with the principles of this invention;

Fig. 5 is a cross-sectional view of an embodiment for achieving the bypass coupling alternate to that shown in Fig. 4;

Fig. 6 is a plan view of another embodiment of the bypass means arranged for coupling with a given preselector filter;

Fig. 7 is a fragmentary sectional view taken along line 7-7 of Fig. 6; and

Fig. 8 is a plan view of an embodiment of the means controlling the bypass communication which may be incorporated with the transmission line of Fig. 6.

Referring to Fig. l, a simple block diagram of a microwave heterodyne receiver is illustrated comprising a preselector filter 1, a mixer 2, and local oscillator 3, an I. F. amplifier section 4, a detector 5, and an intelligence utilizing circuit 6. The units illustrated therein employ "conventional microwave circuits arranged in a manner n such a system, for instance the range of 4400 to 5000 mc., which is employed herein for descriptive purposes. A signal in thisV range received by 'the antenna 7 is coupled "t'o the preselector filter 1, beat with the local oscillator 3, such as a klystron oscillator in the mixer 2. The local oscillator may also operate in the abovementioned frequency range to provide a predetermined intermediate frequency, for instance, in the'range of 30 mc. to 100 mc. The remaining circuits 5 and 6 may be arranged in a predetermined manner to recover and utilize the intelligence signals of the audio type, `or as more often employed,'may be employed in any of the many types of pulse communication links.

Filters employed in the prior art systems inherently made the alignment and tuning of the receiver burdensome and difficult particularly where the filter was extremely selective. Furthermore, the number of tuning adjustments necessitated for the prior art filters are numerous making the tuning procedure of that unit alone very difficult. Therefore, I propose to shunt the filter portion of preselector filter 1 with a bypass line 7 as shown in Fig. 2 to provide simpler tuning of the filter as Well as the overall receiver.

Preselector filter 1 of Fig. 2 comprises a Wideband tunable microwave filter 8 and a shunting line 7. Filter 8 may comprise any conventional waveguide filter Wherein there is preferably an alignment of tunable cavities in a straight section of waveguide, with a separation of a quarter wavelength between adjacent cavities. Preferably the tunable cavities are bounded by fixed susceptance irises, the only tuning necessary therein being that of the resonant cavities by means of micrometer tuning heads controlling tuning probes inserted in the resonant cavities. Shunting line 7 may be coaxial line as shown, or any other suitable transmission line, coupled to the first tunable filter section 9, tunable by means of tuning probe 10, such that a retractable probe 11 may be inserted into section 9 to remove therefrom a portion of the received signal energy without disturbing materially the mode of propagation in the filter, or disturbing the resonance of the cavity into which probe 11 is inserted. This extracted energy is fed through line 7 to the last filter section 12, tunable by probe 13, the bypass signal energy being injected therein by shunt probe 14. This relatively wideband shunted signal is then coupled to the remaining receiver circuits enabling an alignment and tuning of these circuits to a desired signal quickly and easily, since the filter has been shunted and as a result provides a less selective band of signals through which the desired signal may be hunted for, and at the same time filter 1 is tuned to a desired selective band, if it is not already tuned for the selective band.

When this comparatively easy tuning procedure is completed shunt probes 11 and 14 are preferably retracted from cavities 9 and 12, respectively, such that the bypass communication is discontinued. This may be accomplished by withdrawing probe 11 by means of knob 15 and probe 14 by means of knob 16. The means for accomplishing the retraction of the probes may be manual if the filter unit is accessible, or if not accessible a motor driven cam arrangement or other known mechanical linkages, activated from a front panel, may be incorporated to withdraw the probes sufliciently to discontinue the bypass communication. A preferred means of discontinuing the bypass communication is illustrated in connection with Fig. 3 and will be disclosed hereinbelow. Electrical continuity is provided between probes 11 and 14 and the center conductor 17 of line 7 by properly constructed sliding contacts at points 18 and 19, the details of which will be obvious to those skilled in the art.

Retraction of probes 11 and 14 allows the selective 4 characteristic of filter 8 to dominate and thus provides an overall selective microwave receiver. The removal of probes 11 and 14 from cavities 9 and 12 does not alter substantially the alignment of the receiver components as established during the bypass communication, due to the fact that the insertion of probes 11 and 14 is limited to an amount just suiiicient to bypass a portion of the received signal energy and not to materially disturb the propagation mode and resonance of the filter section.

Graphically, Fig. 2a, illustrates the tuning of a receiver having a preselector filter 1 employing the bypass means disclosed herein. When the bypass communication occurscurve 20 illustrates the relative bandwidth of signal energy present for application to the othenreceiver units. This bandwidth allows easier trackingy or hunting of a desired signal and final tuning to this signal. The wide bandwidth accompanied by a reduced amplitude does not materially hinder the alignment procedure. Having the filter sections tuned to a desired selective frequency band prior to the time of discontinuing bypass communication, the discontinuing ofthis communication results in the curve 21 which illustrates the desired selectivity desired for the overall receiver.

Referring to Fig. 3, another embodiment is shown whereby a portion of the received signal energy is removed preceding th`e signal input means to a waveguide filter 22, of any desired conventional form, by means of a probe shunt line 23 similar to that shown in Fig. 2, with thel exception that the Vextracting probe 2 4 ispinserted into a waveguide section 25 to a depth which is just suiicient to remove a desired amount of signalrenergy without disturbingrth-e propagation modeof the filter. This energy is transferred through line 23 and injected by probe 26 into a waveguide section 27 succeeding the signal output means of the waveguide filter 22. The energy is coupled therefrorn'to the remaining circuits of the receiver to achieve the desired alignment in a manner as described in connection with the embodiment of Fig. 2.

Means to control remotely the operation of the bypass communication, whereby after tuning the communications may be limited to the characteristics of filter 22, is accomplished by means of solenoid coils 28 and 29 located in association with the respective shunting probes 24 andj26. The opening of switch 30 discontinues the liow of current through coils 28v andv 29, deenergizing these coils and as a result probes 24 and 26 are retracted from their re. spective lter or waveguide coupling sections, as the case may be, by the action of compressed springs 32'and 33. Insertion of the probes into the respective cavities is vaccomplished by closing switch 30 allowing the current from the D. C. source 31 to flow through coils 28 and 29, energizing these coils in a manner to force theprob'es into the predetermined position inside the coupling sections; Switch 30 is preferably located on a front panel of the equipment incorporating an inaccessible lterunit including the shunting line to provide a convenientmeans to insert and remove the probes from their corresponding waveguide sections.

Fig. 4 illustrates an embodiment of my invention erriployed in conjunction with a tunable coaxialtype microwave filter 34. Coaxial sections35 and v36 are added to a conventional coaxial filter for` the purpose of having coupled thereto the shunt line 37 similar to line 7V shown in Fig. 2. Bypass of the signal'energy herein is yaccomplished by the capacitive coupling between probe 38 and the center conductor 39 of section 35 preceding the signal input means'to filter 34. The bypass signal is returned to the system by capacitive coupling between probe40 and center conductor 41 of section 36 which succeeds the signal output meansl of filter 34. Means for controlling the bypass communication is provided by plunger 42 which is located a half wavelength, or a multiple thereof, from probes 38 and 40. As shown herein plunger@ is located a half of wavelength from probe 38 and a full Wavelength from probe 40 and when moved invvardl it provides a short between the inner and outer conductors of line 37. The bypass communication plunger 42 may be operated by the solenoid controlling means illustrated in Fig. 3, or the alternate methods suggested in connection with Fig. 2. To discontinue or disrupt bypass communication plunger 42 is inserted into line 37 to short center conductor 42a whereby due to the half wavelength relationship probes 38 and 40 are effectively shorted. This means of discontinuing the bypass communication allows the probes 38 and 40 to remain inserted in the path of the signal energy and thereby does not upset, even a slight amount, the overall tuning and alignment achieved during the bypass communication.

Fig. 5 illustrates another embodiment for obtaining coupling between probe 38 and center conductor 39 of Fig. 4. A spring metal loop 43 is secured to probe 38 and a lever 44 may also be secured thereto, extending through a slot 45 of line 37. Coupling between probe 38 and center conductor 39 is achieved by moving lever 44, composed of insulated material, downward to position loop 43 as shown by the dotted line, thus achieving the desired bypass of signal energy from the input to the filter. To remove the coupling between probe 38 and center conductor 39, discontinuing the bypass communication, lever 44 is moved inwardly such that spring loop 43 conforms substantially to the shape of the outer conductor of section 3S, but separated therefrom by dielectric material 46. This removes the loop 43 from the field of the signal energy and discontinues the bypass communication. If desired the lever 44 may be provided with solenoid means 28a and spring 32a similarly as shown in Fig. 3.

Referring to Figs. 6 and 7, another embodiment of the bypass means is illustrated wherein the probes 47 and 48 are inserted permanently into the first and last filter sections of a waveguide filter 49 through openings 50 and 51, respectively. The transmission line 52 interconnecting probes 47 and 48 is of the line-above-ground type. The details of this type of transmission line incorporating controlling means, substantially as herein illustrated and ncluding other embodiments applicable to the present invention, are disclosed in copending applications, Serial Nos. 227,896, filed May 23, 1951, now abandoned; 234,503, filed June 30, 1951, now Patent No. 2,721,312, issued October 18, 1955; and 286,764, filed May 8, 1952.

Transmission line 52 is of the printed circuit type comprising a first or line conductor 53 and a second or ground conductor 54, herein the wall of the microwave filter 49, with a layer 55 of insulating material therebetween. The method of applying the conductive material to the layer of insulation, the width relation between ground conductor and line conductor and the required characteristic of field concentration are disclosed in the aforementioned copending applications.

The tapered sections S6 and 57 of line conductor 53 provide means to match the transmission line characteristic impedance of the system including probes 47 and 48 thus affording properly determined means to couple signal energy from filter 49, as illustrated, and establish the desired bypass communication. This small variation in the width of the line conductor may produce variations in the characteristic impedance of transmission line 52 as desired for impedance matching, but the field distribution with respect to the ground conductor is not materially disturbed, or if disturbed it quickly stabilizes itself to provide the desired bypass communication Without undue signal reflection. For other means of achieving the desired signal coupling between filter 49 and the transmission line 52 reference may be had to the copending application of D. D. Grieg and H. F. Engelmann, Serial No. 279,766, filed April 1, 1952.

The means to control the bypass communication shown in Figs. 6 and 7 comprises a pivot segment 58 substantially identical to the line conductor 53, the segment being pivoted as indicated by pin 59 which may comprise conductive metal or dielectric material. In either case, when the switch portion 58 is in the closed position, indicated by the dotted lines, the pin has substantially no effect on the flow of current along the line.

The location of switch 58 in relation to the coupling probes 47 and 48 is such as to assure the introduction of the selectivity characteristic of filter 49 upon determination of the bypass communication by merely pivoting the switch to the open position shown. To assure termination without loss of signal energy an effective short circuit is required across openings 50 and 51 to remove the coupling of energy by probes 47 and 48. Switch 58 is therefore located a distance from each of the probes 47 and 48, where N is equal to an odd integral. Preferably, the switch 58 occurs a quarter wave length from one of the probes while the distance to the other probe is any odd number of quarter lengths.

Employing a bypass means as herein illustrated, the probes 47 and 48 remain in the path of signal propagation at all times with bypass communication being controlled by discontinuing said communication through the interruption of the continuity of the transmission line and thereby reflecting substantially an effective short to the point where signal coupling is accomplished. By leaving the coupling probes in the path of signal energy any alignment and tuning accomplished during bypass communication will not be disturbed.

Rather than control the bypass communication by means of a switch which interrupts the continuity of conductor 53, Fig. 8 illustrates an attenuator which may replace the switch 58 of Figs. 6 and 7 to provide a means to control the bypass communication. Line conductor 53a is continuous from one coupling proble to the other in this embodiment. Pivoted segment 60 comprises a lossy conductor material capable of absorbing microwave signal energy, the segment being pivoted by means of pin 61. To disrupt the bypass communication attenuator portion 60 is pivoted to the position shown by the dotted lines whereby the bypass energy is absorbed by said attenuator. This, as in the other embodiments introduces the selectivity characteristic of the microwave filter to the system. Upon termination of the bypass communication through means of attenuator 60 a certain amount of energy will be absorbed by said attenuator. This absorbed energy is substantially equal to that energy previously bypassed. Therefore, when the selectivity characteristic of the filter is introduced to the system the amplitude of the signal passing through the system will be decreased by an amount equal to the bypassed energy now being absorbed. Thus the amplitude of the signal passing through the system may be illustrated in Fig. 2a by the currents in amplitude between curves 21 and 62.

Instead of switch 58 or attenuator 60 a shorting screw may be provided in the line 7 located a half wavelength or multiple thereof from the coupling probes 47 and 48.

The embodiments illustrated in Figs. 6, 7, and 8 comprise the subject matter of my copending application Serial No. 302,293, filed August 2, 1952, now Patent No. 2,735,073, issued February 14, 1956.

While I have described above the principles of my invention in connection with specific apparatus and methods, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and inthe accompanying claims.

I claim:

A microwave bandpass filter comprising a hollow waveguide filter of the distributed parameter 'type having signal input and output sections, each of said sections having an opening therein, a coaxial transmission line coupled between said sections with the inner conductor thereof extending through the openings into said sections to provide a bypass communication about said waveguide filter to bypass aporti'onof the signal yenergy from said. input section to said voutput section to reduce Vthe selectivity of the lter during tuning adjustments, and means to interrupt the bypass communication after tuning to introduce the selectivity characteristics of said lter, said bypass coaxial transmission line including a coupling section coupled to each of said input and output sections and a central section coupled between said coupling sections, the center conductor of said coupling sections being disposed in sliding contact relation with the ends of the center conductor of said centralY section, the ends of the center conductor of said coupling sections extending through the opening in said input and output sections to provide a probe type coupling in said input and output sections, said coupling sections being extended beyond the point of sliding contact with the center conductor of said central section, and meansA included in this extended portion to short the inner and outer conductors of said coupling sections to provide a quarter wave choke section therein at the operating frequency ofsaid lter to cooperate in impedance matching,4 and said means to interrupt bypass communication includes means coupled to the center conductors of said'coupling sections to control the probe` lengths thereof extending into said input and output sections.

References Cited in the le of this patent UNITED STATES PATENTS 2,161,593 Rust June 6, 1939 2,218,980 Cawein Oct. 22, 1940 2,226,686 Alford Dec. 31, 1940 2,390,768 Austin et al Dec. 11, 1945 2,498,073 Edson et al. Feb. 21, 1950 2,509,062 Horner May 23, 1950 2,530,979 Matland et al Nov. 21, 1950 2,566,020 Penn Aug. 28, 1951 2,667,597 Bailey Jan. 26, 1954 2,735,073 Grieg Feb. 14, 1956 2,749,523 Dishal June 5, 1956 

